The invention relates generally to electrical current limiting fuses, and particularly to such fuses of the type containing a particulate arc-quenching filler which is bound together into a self-supporting matrix by an inorganic refractory binder. The invention also relates to methods for manufacturing the same.
A current limiting fuse is a special type of fuse which can interrupt a large current at a high voltage in a controlled manner to prevent the occurrence of damaging high voltage transients. Such transients result when the current is interrupted too suddenly. One of the most generally used types of current limiting fuses is the so-called particulate porous matter filled cartridge fuse. In such a fuse, a conductive fusible element, or link, is enclosed in an insulating cartridge, or tube, between two terminal end caps and surrounded by an arc-quenching filler of tightly packed particulate matter, such as silica sand. When the fusible element melts, the resulting arc interacts with the surrounding filler in such a manner that the resistance of the fuse is rapidly, but gradually, increased to the point when the arc can no longer be sustained, thus breaking the circuit in a controlled fashion.
Efforts to improve the performance of such fuses by better control of the interaction of the arc with the filler have led to fuses in which the filler particles are bound together by an inorganic refractory binder, such as colloidal silica. Such a fuse, as well as the manner of making it, is described in detail, for instance, in U.S. Pat. No. 3,838,375 issued 24 Sept. 1974 to Frind et al. and assigned to the same assignee as are the rights to the instant invention.
As is described in the above-cited patent, the filler may be bound together by the admixing therewith either prior to filling or subsequent to the filling of a suitable inorganic binder which is applied in sufficient quantity to coat each individual particle of the filler over substantially the entire surface thereof without substantially diminishing the porosity of the matrix of filler inside the fuse. For the production of large numbers of fuses by the latter approach, the fuse is filled with sand through a fill port in one end cap while being vigorously vibrated, so that the sand is closely packed. Then the fuse is filled with a saturated suspension of colloidal silica in water. Next, the excess suspension is drained out through a drain port in the other end cap. A filter is inserted in the drain port or held against the outside of the end cap over the drain port to restrain the filler. Then the fuse is dried by passing compressed dry air through the interior and baking.
One problem with the present approach of binding the filler is that it frequently takes too long a time to fill the colloidal suspension into the fuse. For draining the excess solution from the fuse 10, it is desirable that the drain port be down and the fill port be up, so that gravity does not work against the movement of the suspension to the drain port. It is desirable, that the drain port be down also from the standpoint of preventing the suspension from running over the tube housing and end caps upon disconnection of the fuse from the fill and drain lines. On the other hand, the filling with suspension would be aided by having the drain port up, but it would be an undesirable hindrance to manufacturing speed to have the fuse with filter down at the drain step, but in the up position for the filling step. Therefore, it has been the practice to orient the fuse with the filter down for the entire production process.
When the filter is in the down position and the suspension is filled into the fill port, some of the suspension immediately makes its way to the filter and wets it, before all the sand in the fuse has been contacted and wetted by the suspension. The wetted filter blocks the rapid passage of air needed to fill the fuse 10 completely with suspension. As the filter is at most about the same diameter as is the relatively small drain port 24, a complete filling of the fuse with suspension therefore can typically require on the order of one quarter to a half hour per fuse, sometimes much longer.